The MonoBehaviour class included in the Unity API, which acts as the base class for most new scripts, offers the SendMessage and BroadcastMessage methods. Using these, you can easily execute functions by name on all components attached to an object. To invoke a method of a class, you typically need a local reference to that class to access and run its functions as well as to access its variables. However, the SendMessage and BroadcastMessage functions let you run functions using string values by simply specifying the name of a function to run. This is very convenient and makes your code look a lot simpler and shorter at the cost of efficiency, as we'll see later. Refer to the following code sample 1-16:
01 using UnityEngine;
02 using System.Collections;
03
04 public class MyClass : MonoBehaviour
05 {
06 void start()
07 {
08 //Will invoke MyFunction on ALL components/scripts attached to this object (where the function is present)
09 SendMessage("MyFunction", SendMessageOptions.DontRequireReceiver);
10 }
11
12 //Runs when SendMessage is called
13 void MyFunction()
14 {
15 Debug.Log ("hello");
16 }
17 }The following are the comments for code sample 1-16:
Line 09:
SendMessageis called to invoke the functionMyFunction.MyFunctionwill be invoked not only on this class but on all other components attached toGameObject, if they have aMyFunctionmember, including theTransformcomponent as well as others.Line 09: The parameter
SendMessageOptions.DontRequireReceiverdefines what happens ifMyFunctionis not present on a component. Here, it specifies that Unity should ignore the component and move on to the next callingMyFunctionwherever it is found.
We've seen that SendMessage invokes a specified function across all components attached to a single GameObject. BroadcastMessage incorporates the SendMessage behavior and goes a stage further, that is, it invokes a specified function for all components on GameObject and then repeats this process recursively for all child objects in the scene hierarchy, cascading downwards to all children.
More information on SendMessage and BroadcastMessage can be found at http://docs.unity3d.com/ScriptReference/GameObject.SendMessage.html and http://docs.unity3d.com/ScriptReference/Component.BroadcastMessage.html.
Note
Reflection
SendMessage and BroadcastMessage are effective ways to facilitate inter-object communication and inter-component communication, that is, it's a great way to make components talk to one another if they need to, to synchronize behavior and recycle functionality. However, both SendMessage and BroadcastMessage rely internally on a C# feature known as
reflection. By invoking a function using a string, your application is required to look at itself at runtime (to reflect), searching its code for the intended function to run. This process is computationally expensive compared to running a function in the normal way. For this reason, seek to minimize the usage of SendMessage and BroadcastMessage, especially during Update events or other frame-based scenarios, as the impact on performance can be significant. This doesn't mean you should never use them. There might be times when their use is rare, infrequent, and convenient and has practically no appreciable impact. However, later chapters in this book will demonstrate alternative and faster techniques using delegates and interfaces.
If you'd like more information on C# and its usage before proceeding further with this book, then I recommend the following sources:
Learning C# by Developing Games with Unity 3D Beginner's Guide, Terry Norton, Packt Publishing
Intro to C# Programming and Scripting for Games in Unity, Alan Thorn (3DMotive video course found at https://www.udemy.com/3dmotive-intro-to-c-programming-and-scripting-for-games-in-unity/)
Pro Unity Game Development with C#, Alan Thorn, Apress